1. Field of the Invention
This invention relates to a sintered aluminum nitride body having a high thermal conductivity. More particularly, it relates to a sintered aluminum nitride body which can be produced through low-temperature sintering, has evenness of color and strength, is inexpensive, and has excellent quality. This invention further relates to a metallized substrate comprising the sintered body.
2. Description of the Prior Art
Aluminum nitride (AlN), having a high thermal conductivity and a low coefficient of thermal expansion, is recently coming to be used as insulating substrates for various electronic parts as a substitute for alumina, which has conventionally been used. Sintered aluminum nitride bodies are expected to be used as substrates for high-power hybrid ICs, because they have excellent electrical insulating properties, a high thermal conductivity, and a coefficient of thermal expansion relatively close to that of silicon.
However, since aluminum nitride is generally sintered at a relatively high temperature of 1,800.degree. C. or higher, there are no sintering furnaces, jig parts, etc. which can sufficiently cope therewith. It is therefore necessary to frequently repair sintering furnaces and frequently discard or replace jigs. Moreover, a larger amount of sintering energy is necessary because aluminum nitride is sintered at high temperatures. Consequently, sintered aluminum nitride bodies are more expensive than sintered alumina bodies. This has been an obstacle to the spread of aluminum nitride.
In general, aluminum nitride is more difficult to sinter than alumina. For sintering it, rare earth metal compounds and alkaline earth metal compounds have mainly been used as sintering aids. In particular, combinations of a rare earth metal compound and an alkaline earth metal compound have been investigated in order to lower the sintering temperature, specifically to enable sintering at 1,700.degree. C. or lower. Representative sintering aids comprise a combination of a calcium compound and an yttrium compound, and many investigations have been made thereon.
For example, Japanese Patent Laid-Open No. 153,173/1987, Japanese Patent Publication No. 49,613/1994, Japanese Patent Laid-Open No. 175,867/1997, etc. disclose a sintered body which comprises aluminum nitride, a rare earth element/aluminum oxide, and an alkaline earth element/aluminum oxide and is obtained from aluminum nitride containing a rare earth metal compound and an alkaline earth metal compound in combination. Japanese Patent Laid-Open No. 190,761/1988, Japanese Patent Publication No. 17,457/1995, etc. disclose a sintered body comprising aluminum nitride as the main component and containing a calcium compound and an yttrium compound as sintering aids.
Furthermore, Japanese Patent Publication No. 7,349/1993 discloses a technique for obtaining a dense sintered aluminum nitride body having a high thermal conductivity by adding to aluminum nitride a nitride of a Group 3A element of the Periodic Table and at least one member selected from the group consisting of the oxides and fluorides of Group 3A elements and the nitrides, oxides, and fluorides of Group 2A elements as sintering aids.
In order to use a sintered aluminum nitride body as substrates for electronic parts, such as IC substrates, the aluminum nitride substrate should be metallized. A known technique for this is a metallization method which comprises applying a paste of a high-melting metal, such as tungsten, molybdenum, or tantalum to a surface of a sintered body or green compact and firing the resultant body at a high temperature in a non-oxidizing atmosphere to form a high-melting metallizing layer.
For example, Japanese Patent Laid-Open No. 29,991/1988 discloses a technique in which a conductor layer comprising tungsten, molybdenum, ZrN, TiN, or the like as the main component and containing at least one member selected from the group consisting of rare earth elements and alkaline earth elements is formed on a green compact comprising aluminum nitride containing a sintering aid comprising at least one member selected from the group consisting of rare earth elements and alkaline earth elements, and the green compact and the conductor layer are simultaneously sintered to obtain an aluminum nitride circuit substrate free from warpage and excellent in thermal conductivity, surface resistance, and tensile strength. Japanese Patent Publication No. 71,198/1993 discloses a method for obtaining an aluminum nitride circuit substrate having satisfactory bonding strength which comprises adding Y.sub.2 O.sub.3 as a sintering aid to aluminum nitride as the main ingredient, sintering the mixture at a temperature as high as 1,800.degree. C., and forming a mixture of a high-melting metal selected between tungsten and molybdenum with at least one member selected from the group consisting of SiO.sub.2, Al.sub.2 O.sub.3, CaO, MgO, BaO, and B.sub.2 O.sub.3 as an adhesion-enhancing agent on the sintered body through firing at 1,600.degree. C. or higher.
However, the post-firing metallization method in which a metal paste is applied to a surface of a sintered aluminum nitride body and then fired unavoidably leads to a cost increase because a high-temperature treatment for metallization is necessary besides substrate sintering. Although a measure for eliminating the above drawback is the co-firing metallization method in which a metallizing layer is formed by firing a metal paste simultaneously with substrate sintering, this method has a problem that the sintered body has a considerable deformation. There is still another problem that since a metallizing layer comprising at least one high-melting metal as the main component has poor corrosion resistance and poor electrical conductivity, it is generally required to be plated, resulting in an increased cost of the metallized substrate.
For eliminating these problems, a metallization method in which gold, platinum, silver, or the like is used has been developed. For example, Japanese Patent Publication No. 76,795/1993 discloses a circuit substrate comprising an aluminum nitride ceramic base comprising aluminum nitride as the main component and containing at least one member selected from the group consisting of yttrium, rare earth metals, and alkaline earth metals and, disposed on the base, a metallizing layer formed from a paste of silver or gold. According to this method, the adhesion strength between the metallizing layer and the base is ensured mainly with the sintering aid incorporated in the sintered aluminum nitride body.
As described above, due to the use of a rare earth metal compound and an alkaline earth metal compound in combination as a sintering aid, it has become possible to conduct sintering at a lower temperatures than conventional ones and to produce a sintered aluminum nitride body having a high density and a high thermal conductivity. As a result, the use of aluminum nitride is spreading gradually as substrates for highly heat-generating semiconductor elements such as power elements.
However, when sintering is conducted using a calcium/yttrium sintering aid, which is the mainstream at present, the resultant sintered aluminum nitride body has drawbacks of unevenness of color, unevenness of strength, unevenness of thermal conductivity, etc. It has therefore been difficult to obtain a metallized substrate free from unevenness of color and satisfactory in tight adhesion when the sintered aluminum nitride body is metallized with any of the aforementioned metals for use in metallization. Therefore, the yields of the sintered aluminum nitride body and the metallized substrate using this are low, and this has been a major cause of the increased prices of aluminum nitride products.
Furthermore, the presence of a large amount of a binder as carbon in an aluminum nitride powder compact during sintering adversely influences the sintering characteristics of the compact. In particular, when a degreasing step is conducted in a nitrogen atmosphere in order to improve the thermal conductivity, carbon remains in a large amount, resulting in a considerable decrease in sintering characteristics. Consequently, the resultant sintered aluminum nitride body has drawbacks that the density, strength, and thermal conductivity thereof are uneven and a metallizing layer is apt to have reduced adhesiveness. Moreover, use of a calcium compound as one component of a sintering aid has a problem that the degreased compact which has been degreased at several hundreds of degrees Celsius prior to sintering is highly hygroscopic and hence develops many cracks within a short time to become unsuitable for sintering.
As to metallized aluminum nitride substrates, as described above, a metallization method in which gold, silver, or the like is used in place of a high-melting metal has been proposed. However, when this method is used for forming a metallizing layer of gold, silver, or the like on a sintered aluminum nitride body, it has been difficult to obtain satisfactory tight adhesion between the metallizing layer and the sintered body base. This is because the adhesiveness of the metallizing layer generally varies considerably depending on the ingredients other than aluminum nitride contained in the sintered body base, their contents, vitreous ingredients contained in the metallizing layer, their contents, etc. The important properties of the metallizing layer as a member of an electrical circuit, such as the stability of resistivity, also vary depending on these factors. Furthermore, when the metallizing layer as a circuit member requires an insulating coating, it is important to match the insulating vitreous layer with the aluminum nitride base and with the metallizing layer.